- GarageBand application for the iPhone (free)
Download the ringtone file to iCloud. Just click this link and then "Add to iCloud Drive"
Open GarageBand application in your phone, click the import button and look for the ringtone file in the iCloud folder, don't open it, just long press it and then click on the share option.
Select the ringtone option and then click Export in the top right corner. Finally when exported, click the option "Use sound as..." and then "Standard Ringtone". Done!
Now you can delete GarageBand application if you want, it takes a lot of space (1.77 GB).
If you want to make any modification to the fade effect you will need to open this folder in a Mac with GarageBand, change the Master Track, and then go to Share > Project to GarageBand for iOS.
You can inspire in that file to make any ringtone increment the volume gradually.